Things to Know
#Genetic Drift
#Gene Flow
#Directional Selection
Review: is a population evolving?
- Re-examine the beetle populations
- color: one locus, 2 alleles
- Across 2 generations, are allele frequencies the same?
- Is this population in Hardy Weinberg equilibrium?

Hardy Weinberg describes a population NOT evolving

Allele frequency change in beetles: Mutation
- Mutations in the beetle color gene, resulting in more brown
- to rare to change allele frequencies on their own
- require synergy with a another driver (e.g., selection)
- Mutation rate = probablity that one allele changes to another via mutation
- mutation rates are generally small (~0.0001%).
- humans = 2.5 x 10-8 mutations nucleotide-1 generation-1
- negligible change from one generation to the next
- Mutation ARE the ultimate source of genetic diversity

Allele frequencies change in beetles: Genetic Drift
- Genetic drift describes how allele frequencies may fluctuate unpredictably across generations
- happens in every population
- pronounced in small populations
- extremely important evolutionary “force”
- Reduces genetic variation through the random loss of alleles
- In a small population, beetles are green or brown
- several green beetles get stepped on by Dr. C
- next generation has more brown beetles, by chance
- did it matter if alleles were dominant or recessive? or better suited? or helped with mating?

Hypothetical coin flips to choose 2 alleles
- Expected frequencies of alleles are just mathematical probability (not certainty)
- In small populations should we expect the allele frequencies to remain identical across generations?

Mechanism of Genetic Drift: Founder Effect
- Founder Effect: when a few individuals become isolated from a larger population and establish a new population
- some alleles are kept, others are lost
- allele frequencies likely change by chance
- Occurs frequently on islands
- what could happen with bad alleles?



Mechanism of Genetic Drift: Bottleneck Effect
- Bottlenecks occurs when an event drastically reduces a population size, likely altering allele frequencies
- sudden change in the environment
- fire, flood, invasive species, etc.
- The new gene pool may be different from original population
- If the population stays small, genetic drift could increase across populations
- black robins on Chatham island

Bottlenecks, species habitat loss, and conservation

Allele frequencies can change through Gene Flow

- Gene flow: exchange of alleles between 2 or more populations
- aka ‘Gene migration’
- like mutations but way more frequent
- Occurs when individuals migrate and mate successfully
- brown beetles enters population
- changes allele frequency for color
- Gene flow can be good or bad
- may increase genetic diversity
- may bring harmful alleles
- Does gene flow tend to make populations more or less unique?

Allele frequencies change in beetles: Natural Selection

Only natural selection causes adaptive evolution!
- ‘Sorting’ of alleles that favor reproductive success
- leads to adaptation at population level
- Certain traits lead to greater Relative Fitness
- individuals with beneficial traits (alleles) will contribute more to gene pool
- better survival/reproduction compared to others
- More individuals in a population with greater relative fitness means….
- Natural selection is a ‘moving target’…WHY?

Natural Selection: selection of heritable traits

Natural Selection: Directional selection of heritable traits

Natural Selection: Diversifying selection of heritable traits

Natural Selection: Stabilizing selection of heritable traits
